The present invention relates to a linear actuator.
Linear actuators, for instance hydraulic rams, have many applications, in lifting, extending linkages and equipment etc. Electric linear actuators are known for applications where a source of pressure fluid is not available.
Such linear actuator is described in U.S. Pat. No. 4,934,203 comprises: a motor housing having one end fitting, a motor housed in the motor housing, a reduction gearbox housed in the housing, the motor being connected to a input of the gearbox, and a telescopic arm, having an outer tube attachable to the motor housing, an inner tube having another end fitting, an extension screw having a driven end journalled at the motor housing end of the outer tube and engaging a nut in the corresponding end of the inner tube, rotation of the screw causing the inner tube to extend or retract with respect to the outer tube,
In hostile environments, such as onboard fishing vessels, electric actuators have a shorter life than in less onerous environments. Failure of actuators can have a serious effect on the operation of a fishing vessel, so that a supply of spare actuators is desirable. Since a variety of different sizes of actuators are required, for different hoists etc., the supply of spare actuators must include the different sizes, despite space being at a premium.
The object of the present invention is to provide an improved actuator whose the main components are separately replaceable, whereby spares inventory limitations are eased.
According to the invention there is provided a linear actuator comprising:
a motor housing having one end fitting, a motor housed in the motor housing, a reduction gearbox housed in the housing, the motor being connected to an input of the gearbox,
a telescope arm, having an outer tube attachable to the motor housing, an inner tube having another end fitting, an extension screw contained within the telescopic arm and the motor housing and having a driven end journalled at the motor housing end of the outer tube and engaging a nut fast at the proximal end of the inner tube, rotation of the screw causing the inner tube to extend or retract with respect to the outer tube,
characterised in that the linear actuator further comprises:
an engageable connection of an output of the reduction gearbox and the driven end of the extension screw for transmitting drive from the gearbox to the extension screw for extension and retraction of the inner tube,
a detachable connection of the outer tube to the motor housing, the arrangement being such that attachment of the outer tube to the motor housing and engagement of the extension screw with the gearbox prepares the linear actuator for use.
In event of failure in use, either the motor in the housing or the telescopic arm are likely to have failed without failure of the other. Thus the actuator can be simply repaired by replacement of the relevant portion.
Further, where a variety of different lengths of telescopic arms, possibly with a variety of differently pitched screws, are used on a single ship for instance, spare arms can be carried and fitted to commonly sized motor housings. A smaller supply of these can be carried in accordance with the service life in the environment.
Preferably, the engageable connection is an axially engageable dog clutch. This can comprise a clevis and a spigot with a transverse pin engageable in the clevis, the clevis being connected to one of the extension screw and the gearbox output and the spigot and pin being connected to the other. To allow for possible slight misalignment, the transverse pin can have a pair of rollers arranged on it, the rollers being on respective opposite sides of spigot.
In the preferred embodiment, the detachable connection of the telescopic arm and the motor housing is a screw connection. The latter preferably incorporates an O-ring seal for sealing the telescopic arm to the motor housing.
For control of stroke, position, velocityxe2x80x94by electronic circuitryxe2x80x94the output of the gearbox is preferably provided with a rotation counter. Provision of the counter at the gearbox output enables the count to be of a smaller number of revolutions, by the gearbox ratio, than if the counter were arranged to count the revolutions of the motor.
Preferably, the linear actuator includes a seal between the inner tube and the outer tube, the seal leaving the tubes being free to rotate relatively about a common axis, the end fittings being adapted to restrain relative rotation for extension and retraction of the telescopic arm.
Preferably, the nut at the end of the inner tube and having the extension screw engaged therein is of generally weaker material than the other components, whereby it forms a weak link in the event of overloading of the actuator.